Cannonballs sold by the NPC at the entrance of the Dwarf underground are sold for 5gp each.

They should be higher price(back in 2006 they were averaging 150-200gp each when sold by players) and the cannonball mould should also be sold in this shop for 5gp.

I'm not even sure it was possible to buy cannonballs from this shop in 2006.